I see what you mean. ITunes doesn't give any indication of the actual server location of each podcast. Without that, it's challenging to discover which of your podcasts is the one causing the trouble. In iTunes, do any of your podcasts have and alert symbol next to them? That might be the only tip you will have as to which podcast is causing the problem.
As for background on Mevio, they recently exited the podcast hosting business. You can read about that here.
http://davidjackson.org/2014/04/mevio-failed/
The iTunes store is only a directory of odcasts, it doesn't host the podcast files for download. It just has pointers to the files on whatever server the author is using to host the files. When that server stops accepting connections (like Mevio did), your downloads will fail. Unfortunately, iTunes can't fix this. You have to get in touch with the podcast author, and if they have moved their podcast to a new hosting server, then they need to update their entry in the iTunes store to point to the new host. If their new host is up and running, but iTunes store isn't updated yet, you should be able to subscribe to the new server manually. iTunes has an option to subscribe to a new podcast manually, so you'd just need the new URL to paste into the requisite field.